Neutron detection is a challenging task due to the fact that neutrons have no distinguishing charge, in contrast to alpha particles, beta particles, or excited electrons from gamma ray interactions. Typically, neutrons are detected through nuclear reactions, such as absorption or scattering reactions. However, those nuclear reactions tend to cause energy identification of the neutrons to be lost, thereby making neutron spectroscopy difficult to realize. Several methods have been proposed to measure neutron spectra from unknown (or known, but uncalibrated) sources. Some of the methods are briefly reviewed below.
The “Bonner Sphere” detection method consists of a small neutron sensitive scintillation detector inserted in a high-density polyethylene (HDPE) ball. The system consists of a set of balls, ranging from 3 inches in diameter up to 14 inches in diameter. A measurement is made with each ball, one after the other, under identical operating conditions. From known response curves, the neutron spectrum can be back-calculated from the data through unfolding techniques. Most changes are greatest for neutrons under 1 MeV. However, for neutrons greater than 1 MeV, the neutron detection response curves for the Bonner sphere set are very similar. In addition, the mass of the spheres and the method used makes Bonner spheres impractical for fast and/or portable neutron spectroscopy.
A nuclear plate camera can be used to discern neutron energies. The system consists of a HDPE radiator fastened to the front of a vacuum cylinder or box. Fast neutrons interacting in the plastic eject recoil protons, with the most energetic protons being completely forward scattered. A film plate, set at 10 degrees from normal, is located at the end of the box. A series of collimators ensures that only forward scattered protons reach the film plate. Proton interactions in the film produce a measurable track, the length of which correlates to the forward scattered energy of the proton, and therefore the neutron energy. The film must be developed; therefore, immediate interpretation of the results is difficult or impractical.
The 3He device depends upon the 3He(n,p)3H reaction, with a Q=0.764 MeV. Fast neutrons absorbed in the 3He gas produce energetic charged particle reaction products with total energy equal to the initial neutron energy plus 0.764 MeV, thereby allowing for the original neutron energy to be calculated. Additionally, fast neutron recoils off of the 3He gas produce a noticeable recoil peak at 75% of the initial neutron energy, giving a second method to check the initial neutron energy. Gas recoil detectors rely solely upon fast neutron scattering reactions; hence the devices typically use hydrogen, or a hydrogen gas mixture, or helium. The recoil peak established on a pulse height spectrum allows for the calculation of the initial neutron energy.
The proton recoil telescope system is similar to the nuclear-plate camera, except charged particle detectors are set at known angles with respect to the HDPE radiator. Hence, the energy deposited in the charged particle detector, along with the angle of incidence, yields the initial neutron energy. However, the concept relies on the fact that the initial trajectory of the fast neutron is known; hence the origin of the fast neutrons must be given.
Time of flight spectrometers rely upon the velocity and energy correlation with neutrons. A set of “choppers”, slotted cylinders with variable angular velocities, are set apart by a significant distance. The rotating slots are synchronized such that neutrons of a pre-established velocity can pass through both choppers, but the second chopper will block slower or faster neutrons. A detector is located beyond the second chopper, which will detect only those neutrons that can pass through the entire apparatus. By adjusting the chopper angular velocities for each measurement, a spectrum of the neutron field can be measured provided that the direction from which the neutrons are coming is known. Typically, time of flight spectrometers tend to be relatively large and lacking portability.
Plastic scintillators rely upon (n,p) reactions to produce measurable scintillation light. The recoil protons produce scintillation light as a function of energy. The scattered neutron may lose all of its energy in a single collision, thereby giving all of its energy to the recoil proton. Alternatively, the neutron may lose its energy through a series of scatters, thereby distributing its energy to many protons. The light is measured with a photomultiplier tube, or some other light-sensing device. Some problems with plastic scintillators are: (a) their light emission spectrum is non-linear with respect to energy deposition and particle mass; (b) they are fairly insensitive for proton recoils with energy less than 1 MeV; and (c) the scintillator mass (volume) required to stop energetic neutrons is significant, hence Compton electrons excited by gamma ray interactions in the material can contribute to background noise.
The capture-gated neutron spectrometer utilizes a plastic scintillator that has been doped with 10B. Recoil reactions (n,p) occur rapidly and produce scintillation light from recoil protons with about 50 ns, which can yield the total energy of the original neutron provided that all of the neutron energy is absorbed in the scintillation block. Thermalized neutrons can diffuse to a boron site, which can take several microseconds, after which another scintillation flash will be observed from the 10B(n,α)7Li reaction (Q=2.31 MeV). If the second flash occurs, equivalent to 2.31 MeV energy deposition, then the first flash is indicative of the original neutron energy. If a second flash does not occur, then the first flash is ignored as having been produced by partial energy deposition of the neutron. The system suffers from the non-linear light emission attributes of plastic scintillators. Furthermore, completing processes with carbon scattering in the scintillator tends to enhance the non-linear response.
Five classes of wide energy range and non-time-of-flight neutron spectrometers have emerged over time, including: (1) single detectors enclosed by multiple neutron interaction materials; (2) multiple detectors individually enclosed by different neutron interaction materials; (3) multiple detectors collectively enclosed by a single neutron interaction material; (4) single position sensitive detectors enclosed by multiple neutron interaction materials; and (5) instruments which comprise a combination of elements from the first three.
In the first class, a combination of boron and/or cadmium, lead or tungsten, and high hydrogen concentration material (usually, high density polyethylene [HDPE]) are used as filters, spallation centers, and moderators to provide ever better response up to ones of GeV incident neutron energy (e.g., Can berra's SNOOPY or Thermo's SWENDI-II). These instruments are known colloquially as the Andersson-Braun (AB) type. The downside of this approach is that the total mass is high (usually >10 kg) and the intrinsic detection efficiency is low.
In the second case, multi-band detectors usually tune three or more detectors to the thermal, epithermal, and fast neutron spectrum ranges but without extraneous moderator. The implication here is a lightweight instrument (e.g., Ludlum's PRESCILA). However, the average energy resolution over the thermal to fast range is consequently the poorest of the five methods because of severe over or under response in the bands not covered.
The third method employs many individual thermal neutron detectors in an HDPE or comparable moderating matrix to provide a depth dependent intensity of thermalized neutrons that yields both the highest efficiency and lowest average dose- and dose-rate-error of the above methods. The shortfall of these instruments is their large moderating volume (usually a 30 cm diameter sphere) needed to accommodate tens-to-hundreds of individual detectors, rendering a non-portable device (>40 lbs with electronics).
The fourth method utilizes a single position sensitive detector enclosed by moderator and filter materials as an improvement to the classical long counter. This detection scheme suffers from large moderating volumes and low intrinsic efficiency due to high neutron absorption in the moderator and/or scattering of neutrons outside the detector volume.
There are only a few examples of the fifth class which utilize a combination of elements from the first three. Like the second class, these dosimeter schemes use a superposition of responses, but they incorporate an important improvement in that the overlapping energy response bands are continuous providing for a much better dose equivalent match. The downside is again the large total volume and low intrinsic efficiency.
Passive identification and/or differentiation of spontaneous fission (e.g., 252Cf), radioisotope (α,n) (e.g., AmBe), and/or spallation (e.g., cosmic-ray induced) neutrons sources represents a significant challenge. High total or intrinsic neutron detection efficiency over the specified energy range is important with regard to collection time and in being sensitive to the bare, filtered, and/or moderated incarnations of the above identified neutron sources. Spectroscopic resolution is important as it provides a means of deconvolving, identifying, and/or verifying known and unknown neutron sources. Portability is important with regard to man-based searches. For example, the SNOOPY NP-2 neutron REM meter currently used onboard nuclear Navy ships weighs 22 lbs. and prevents sailors from performing their job as well as they could if a lighter REM meter were made available. Direct or effective insensitivity to photons, which could swamp out or be recorded as false positive neutrons, is especially prudent given the ease through which gamma emitters naturally exist, are omni-present with any neutron source, and can intentionally be placed as a red herring by those wishing to thwart neutron presence and properties. Determination of absolute neutron flux is useful toward verification of source strength and in displaying the real-time ambient neutron dose equivalent. Ambient neutron dose equivalent is an important measure of absorbed dose, weighted for the energy(ies) of the absorbed neutron(s). Some real-time portable REM meters yield incredible error, especially in the epithermal neutron energy range. Given not only the dearth of 3He, but the flux and spectral variance of neutrons and photons, there exists a need for neutron sensitive instruments that cure at least some of the foregoing deficiencies and enable performance attributes desired in the art.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention.